Why do some snakes have venom instead of just strong teeth?

Snake venom is a modified saliva containing proteins and enzymes that can disrupt nerves, blood clotting, or tissues. For many species, venom lets them subdue prey much larger or more dangerous than they could safely overpower otherwise. It also starts breaking down tissues before the meal is swallowed, which can aid digestion. Venom evolved independently in different snake groups over long evolutionary time, making it a highly successful hunting tool. Not all snakes are venomous, but in venomous species it is a major adaptation for feeding and defense.